At last year's SC2002 conference in Baltimore, Berkeley Lab racked up
its third straight win in supercomputing's annual Bandwidth Challenge
with a data-gobbling visualization of colliding black holes. When it comes
to remote scientific visualization, says Wes Bethel with a smile, "we're
the kings."

Now Bethel and John Shalf of the Computational Research Division's Visualization
Group have followed up their SC2002 success by writing a good chunk 
including the guest editorial  of the March/April, 2003, issue of
IEEE Computer Graphics and Applications, which is devoted to graphics
on the Grid.

Wes Bethel and John Shalf tackle the Bandwidth
Challenge at SC2002.

"The Grid integrates all sorts of devices, services, and resources,
not just computers," Shalf says. Operating inside a specialized world
of research, the Grid hopes to do for the tools of science, from computers
and data-storage systems to instruments like telescopes, electron microscopes,
seismographs, synchrotron beam lines  even oceangoing floats that
report via satellite  what the World Wide Web did for desktop PCs.

But Berkeley Lab's bandwidth champs are far from complacent about the
future of visualization on the Grid. In fact, Bethel and Shalf argue,
there is a "wide gulf between current visualization technologies
and the vision of global, Grid-enabled visualization capabilities."

Their editorial focuses on the gulf between enthusiasm bordering on hype,
on the one hand, and on the other, the tools that can actually be used
by scientific researchers in their day-to-day research activities. In
the process, the editorial highlights several of the most important technical
challenges facing the Grid visualization community.

Bethel and Shalf describe what many envision, a future in which "large,
multidisciplinary teams scattered around the world" can work with
sophisticated visualizations powered by a huge inflow of information to
their individual desktop machines. They sketch a scenario of an imaginary
geophysical and materials-science team using real-time, interactive models
that integrate input from experiments on the molecular scale, seismograms
of natural and induced earthquakes, phone calls from the field, and all
kinds of other data collected by a "vast network of sensors."

Dr. Jane's dream Grid: a seamless web connects the principal researcher
and her collaborators around the world with disparate sources of
data, processing facilities, and interactive visualization capability.

"The vision is a noble one," says Bethel, "but there is
a huge gap between it and what can be done at present." One issue
is what Bethel calls the "Tower of Babel" problem: "A major
objective of the Grid is a uniform means of communication. But in order
for Grid components to be able to communicate, they must all speak the
same language, using the same conventions. In the visualization world,
there are many different data file formats and grid types, and no widespread
agreement about how to go about having disparate software components interact
with one another."

A closely related concern is security. "The Grid couldn't work if
a user had to log into all these sites separately," Shalf remarks.
Yet in working toward secure sign-ons "the Grid community has spent
too much time on getting different components to talk to each other"
 at least from the standpoint of effective visualization systems.

Lossy versus bossy

Part of the problem is that "historically, network specialists have
a fear of lost data." In a major article in the same issue of Computer
Graphics and Applications, which draws on their experience with the
SC2002 Bandwidth Challenge, Bethel and Shalf characterize the data-loss
issue as one of balancing "the competing interests of interactivity
and fidelity"  determining when absolute accuracy is needed
and when it is not.

"The visualization community has long worked with missing data,"
Bethel notes. "So John and I asked whether it is hypocritical to
insist that a visualization system preserve every single bit in the datastream,
without loss. After all, MPEG movies and JPEG images are lossy, yet are
widely accepted within the scientific community. The challenge is to have
predictable behavior with loss in the data used to create the visualization,
not just with lossy compression of images resulting from the visualization
process."

In one of the two broad approaches that characterize present systems,
the visualization is first performed on a single server, then sent to
the client  an approach that can handle large datasets but stonewalls
interactivity. The other approach is to transfer subsets of data that
are assembled on the client's desktop  which is fine for interactivity
but can't keep up with the ever-increasing size of scientific data sets
or the limitations of finite network bandwidth.

Both these approaches preserve the integrity of the data as it travels
the internet. Neither works if large datasets and interactivity are needed
simultaneously. Volume rendering in full 3-D uses up a lot of computing
power and bandwidth; some systems take hours to render a single frame.

Yet, says Shalf, "Loss of data may not have much of an impact if
it doesn't lead to misinterpretation." The volume-rendering program
named Visapult, whose development was spearheaded by Bethel, was designed
to work quickly over the network using a combination of parallelism, pipelining,
and novel "latency-tolerant" visualization and graphics algorithms.